1. Technical Field
The disclosed subject matter relates to a liquid crystal display of a vertical alignment type. In particular, the presently disclosed subject matter relates to a liquid crystal display having a liquid crystal layer with a retardation of about 600 nm or more and an effective viewing angel compensation structure therefor.
2. Description of the Related Art
In recent years, viewing angle compensation means having a variety of optical characteristics have been proposed for vertical alignment type liquid crystal displays. In particular, examples of such viewing angle compensation means include viewing angle compensation plates.
Japanese Patent Publication No. Hei 7-69536 discloses a liquid crystal display having a vertical alignment type liquid crystal cell and first and second polarizing plates which are cross-Nicol disposed on respective sides of the liquid crystal cell. In this liquid crystal display, viewing angle compensation means is provided between the liquid crystal cell and any one of the first and second polarizing plates.
In the disclosed liquid crystal display, the viewing angle compensation means comprises a viewing angle compensation plate having three main refractive indices. In this instance, one of the main refractive indices is smaller than the other two main refractive indices, and an axis corresponding to the smallest main refractive index is parallel to the normal direction of the compensation plate.
In addition to this, the patent document discloses that it is effective to utilize a negative uniaxial compensation plate, so-called “C-plate,” as a viewing angle compensation plate, which has an optical axis being parallel to the normal direction of the compensation plate. Further to this, the patent document also discloses that it is effective to utilize a biaxial compensation plate, so-called “biaxial plate,” as a viewing angle compensation plate, which is a medium having biaxial optical anisotropy and which has a minimum main refractive index with its axis being parallel to the normal direction of the compensation plate.
The invention of Japanese Patent No. 3330574 includes viewing angle compensation means which is provided between the liquid crystal cell and one polarizing plate as in the previous case. This viewing angle compensation means utilize a viewing angle compensation plate, so-called “biaxial plate,” which has three main refractive indices where one of the main refractive indices is smaller than the other two main refractive indices, and an axis corresponding to the smallest main refractive index is parallel to the normal direction of the compensation plate. This biaxial plate is arranged such that larger one of the other two main refractive indices, namely the delay phase axis in the in-plane direction of the viewing angle compensation plate, is substantially parallel to or perpendicular to the absorption axis of the adjacent polarizing plate. In this configuration, the patent document discloses that the retardation in an in-plane direction of 120 nm or less is advantageous.
The invention of Japanese Patent No. 3027805 includes viewing angle compensation means which is provided between the liquid crystal cell and one polarizing plate as in the previous cases. The viewing angle compensation means is composed of a so-called A-plate (which has an optical axis in an in-plane direction and is a positive uniaxial compensation plate whose refractive index anisotropy is positive) and a so-called C-plate (which has an optical axis being parallel to the normal direction of the compensation plate and is a negative uniaxial compensation plate whose refractive index anisotropy is negative). In this viewing angle compensation means, the A-plate, the C-plate, and the polarizing plate are arranged in this order from the liquid crystal cell side. In this instance, the patent document discloses that the retardation of the A-plate is advantageously set to 120 nm or less.
Japanese Patent Laid-Open Publication No. 2000-19518 discloses a vertical alignment type liquid crystal display in which liquid crystals are oriented in an axial symmetric manner in each of liquid crystal areas when a voltage is applied thereto. It is also disclosed that this liquid crystal display is advantageously provided with a biaxial plate as disclosed in Japanese Patent Publication No. Hei 7-69536 or the publication of Japanese Patent No. 3330574, or a viewing angle compensation plate prepared by stacking the A-plate and the C-plate as disclosed in Publication of Japanese Patent No. 3027805.
As shown, the inventions disclosed in the publications of Japanese Patent No. 3330574 and No. 3027805 and Japanese Patent Laid-Open Publication No. 2000-19518 have proposed the respective optical compensation plates which are configured based on the principle of the viewing angle compensation means provided to the liquid crystal display of vertical alignment type disclosed in Japanese Patent Publication No. Hei 7-69536 and are further improved by setting certain viewing angle compensation conditions.
Here, Japanese Patent Publication No. Hei 7-69536 does not specifically discuss the product of the birefringence of the liquid crystal and the cell thickness, namely, the retardation of the liquid crystal layer. In this regard, the publications of Japanese Patent No. 3330574 and No. 3027805 and Japanese Patent Laid-Open Publication No. 2000-19518 each disclose that the advantageous effect can be obtained within a certain range of retardation values. Specifically, the publications of Japanese Patent No. 3330574 and No. 3027805 disclose that the retardation value of the liquid crystal layer is advantageously in the range of from 80 nm to 400 nm. Japanese Patent Laid-Open Publication No. 2000-19518 discloses that the retardation value of the liquid crystal layer is advantageously in the range of from 300 nm to 550 nm.
Since the liquid crystal displays disclosed in the publications of Japanese Patent No. 3330574 and No. 3027805 and Japanese Patent Laid-Open Publication No. 2000-19518 are active matrix type liquid crystal displays that are typically represented by TFT-LCDs, the retardation values are specifically limited as described above. In other words, the above-referenced publications do not consider cases in which a liquid crystal layer in liquid crystal display has a larger retardation value.